The present invention is directed to a laminate tile pole piece for an MRI system, a method manufacturing the pole piece and a mold used for bonding a pole piece tiles.
In recent years, a so-called laminate tile pole piece has been developed for an MRI. In view of such development, a plan view of the laminate tile pole piece is shown in FIG. 1A and a side view is shown in FIG. 1B. The pole piece 10 comprises a soft iron circular base plate 11, a soft iron ring 12 around the circumference of the base 11 for directing the magnetic flux into the gap between magnets, soft ferrite laminate tiles 13 and 14 and a soft iron core 15 for mounting a gradient magnetic coil. The laminate tiles 13, 14 and the core 15 comprise the pole piece face. The laminate tiles 14 in the center of the base plate 11 have a greater thickness than laminate tiles 13 at the periphery of the base plate 11 to form a convex protrusion 16. The convex protrusion 16 improves the uniformity of the magnetic field.
However, the prior art laminate tile pole piece has several disadvantages. First, most laminate tiles 13, 14 have a square or rectangular shape. However, the base 11 and the ring 12 have a circular shape. Therefore, in order to fit square or rectangular tiles into a circular opening, edge filler tiles 13A are required. As shown in FIG. 1A, each edge filler tile 13A has a unique, odd shape to allow the peripheral tiles 13 to completely fill the circular base 11 and ring 12. Each edge filler tile 13A must be formed separately from other tiles 13 to create its unique shape. This increases process costs and complexity.
Second, the protrusion 16 also has a circular shape, as shown in FIG. 1A. Therefore, in order to arrange the square or rectangular central tiles 14 in a circle, edge filler tiles 14A are required, as shown in FIGS. 1A and 1B. The edge filler tiles 14A also have a unique, odd shape to allow central tiles 14 to form a circular protrusion 16. Furthermore, in order to allow central tiles 14 to fit with the peripheral tiles 13 without leaving gaps, edge filler tiles 14A also must have two different thicknesses, as shown in FIG. 1B. Each uniquely shaped edge filler tile 14A must also be formed separately from other central tiles 14. This further increases process costs.
Third, the prior art methods of attaching individual laminate tiles 13, 14 to the base 11 involve placing the individual tiles onto the base and then poring epoxy over the tiles. However, the epoxy may flow out of the base and coat portions of the pole piece not intended to be coated by epoxy. Some tiles may also be insufficiently coated with the epoxy because the epoxy is not supplied under pressure. These tiles may become delaminated during MRI use. Furthermore, it becomes very difficult to achieve the optimum height for the protrusion 16 by manually stacking tiles 14 onto a base 11 because of human error. Therefore, different pole pieces manufactured by the prior art method suffer from poor reproducibility and have different performance characteristics due to a variance in the height of the protrusion.
In view of the foregoing, it would be desirable to decrease the manufacturing process cost and complexity by forming a laminated tile pole piece that does not contain oddly shaped edge filler tiles. The present invention provides a pole piece for a magnetic resonance imaging (MRI) system. The pole piece comprises a plurality of trapezoid or annular sector shaped tiles arranged in a plurality of concentric annular arrays.
It would also be desirable to obtain a reproducible and accurate laminate tile pole piece manufacturing process. The present invention provides a method of making a pole piece. The method comprises placing a plurality of tiles into a mold cavity, filling the mold cavity with an adhesive substance to bind the plurality of tiles into a unitary body, removing the unitary body from the mold cavity and attaching a second surface of the unitary body to a pole piece base to form a first pole.
The present invention also provides a mold containing an non-uniform cavity surface for forming a laminate tile pole piece for an MRI system. The mold is made by performing a simulation of magnetic flux density between a first magnet of the MRI system and a second magnet of the MRI system, determining an optimum contour of a first surface of the pole piece based on an optimum value of the magnetic flux density between the first magnet and the second magnet and forming the mold cavity surface contour as a substantial inverse of the optimum contour of the first surface of the pole piece.